Chapter 1: Our Picture of the Universe

Before it became known that the earth is sphere shaped, many people believed that the earth was flat. They thought that if you sailed to the end of the earth, you would fall off the edge. We now know that the earth is in fact sphere shaped and that it rotates around the sun. In contrast, the sun rotates around other stars in our galaxy.  In 340 B.C., a Greek philosopher by the name of Aristotle wrote a book explaining that the earth was not flat. He discovered that the lunar eclipse occurred when the earth came  between the sun and the moon. He also observed the sphere-shaped shadow that the earth casted on the moon. It turns out that Aristotle was correct about the shape of the earth. However, he also believed that the earth was stationary and that the stars were moving. We have since learned that the solar system revolves around the sun, not the earth. Ptolemy made the following model depicting Aristotle's idea of the universe.

http://csep10.phys.utk.edu/astr161/lect/retrograde/aristotle.html

In the early 14th century, a Polish priest named Nicholas Copernicus proposed that the sun was stationary and that the planets move around the sun in different orbitals. Galileo Galilei, an Italian astronomer, found proof supporting Copernicus's theory when he was observing Jupiter with a telescope. He discovered that Jupiter had moons that were orbiting around it. This proved that the earth was not the center of the universe. A German astronomer named Johannes Kepler altered Copernicus's theory by explaining that the planets moved around the sun in ellipses instead of circles. In the late 15th century, Sir Isaac Newton developed complicated mathematical equations to describe Kepler's ellipse theory. In this paper he described the magnetic and gravitational forces responsible for the planets' paths around the sun. Newton also did the soon-to-be famous gravitational experiment involving and apple being pulled to the center of the earth by gravity. The following image depicts the Copernicus model.

http://www.worldculturepictorial.com/blog/content/solar-system-moving-100000-mph-faster-thought-15-speed-increase-translates-doubling-mass-mil

Newton realized that according to his theory of gravitation, the stars should attract one another and should therefore be moving towards one another. This could be true if there were a fixed amount of stars in a fixed amount of space. However, if there were an infinite amount of stars in an infinite space, there would be not central point for the stars to move to. It turns out that new stars are formed every day and just as many die in a day. Therefore, an exact estimate of the number of stars in the universe is beyond count. Back then, no one considered the idea that the universe was constantly expanding. People supporting Newton's theory tried to fix the problem by proposing that there was balance between attractive forces between neighboring stars and the repulsive forces of stars further away. Many new questions of how the universe was created arose from these beliefs. The object of science was to develop a single theory to explain everything about the universe. Today, the general theory of relativity and quantum mechanics are the two leading theories of the universe. The general theory of relativity describes gravitational force and the large structure of the universe. Quantum mechanics explains the universe by looking at its smaller components. Although both are widely accepted, astronomers believe that they can not both be correct. The main objective of this book to to find a theory that will support both the theory of relativity and the theory of quantum mechanics.

Introduction

Many people live their lives without understanding a lot about the universe around us.  There are many factors that play a huge role in our society but go unnoticed. In this book, Stephen Hawking explains the range of topics associated with cosmology, such as the Big Bang, black holes, light cones, and other astronomical phenomenons.